Soil conditions, fertilizers affect the sweetness of Vidalia onions

Published on 08/05/21

By Maria M. Lameiras for CAES News

With high-margin vegetable crops such as Vidalia onions — where an increased yield on a smaller plot can mean much higher profits — determining whether fertilization rates affect quality and yield could make the case for investing in precision agriculture technology to produce a better-quality product.

Researchers at the University of Georgia Agricultural and Environmental Services Laboratories (AESL) want to make sure that the Vidalia onions you buy every year are as sweet as you expect them to be.

The official vegetable of Georgia, Vidalia onions are known for being sweet because of the low sulfur environment of the sandy Coastal Plain soils in the southern part of the state where they are grown, said Daniel Jackson, manager of the Crop Quality Lab, a unit within AESL in the College of Agricultural and Environmental Sciences.

With around 30 approved varieties of Vidalia onions grown in Georgia, each with slightly different flavor characteristics, Jackson and his fellow researchers are trying to determine how to ensure consistently sweet onions — in every field, under any growing conditions, in every harvest year.

“We're talking about 20 counties where Vidalia onions are grown, all of which have similar soil types, but there's a lot of variability, even within a single field. The main point of this research is to determine if the in-field variability of soil characteristics is enough to change onion pungency or onion flavor profile,” said Jason Lessl, program coordinator of the AESL’s Soil, Plant and Water Analysis Laboratory.

“What we've seen is that these soils are sandy on the surface — the top 8 to 20 inches — almost like beach sand. Below this sandy soil is a thick clay layer that can loosely absorb certain nutrients, like sulfur. As the sulfur moves down through the soil profile, it can build up in the clay layer, which then provides a large reservoir of sulfur for the onion. We hypothesize that this pool of sulfur in the clay can be problematic for onion flavor quality, the shallower it is, since the onion roots will have easier access to it,” he said.

Many fertilizer inputs contain sulfur, and while sulfur is a critical nutrient for all plants, including onions, surplus sulfur in the soil is readily absorbed by onions. With sandy soils, sulfur leaches deeper into the soil, distributing the nutrients and preventing excess accumulation. A shallow claypan —a dense layer of the subsoil with a much higher clay content than the overlying material— limits or slows the downward movement of water and some nutrients, like sulfur, through the soil making them more readily available for uptake into the plants.

“That’s when you get pungent onions,” Jackson said. “Sulfur is a mobile nutrient, and readily moves through sandy soils. However, sulfur binds to clay, so if you apply a lot of sulfur to soils with high clay content, the sulfur can stick around and the excess is accumulated by the onions, which affects the onion flavor. The big picture purpose of this research is to figure out how important this process is to producing sweet onions and how much it varies within a single field or among fields within the region due to differences in soil type.”

Working with three different fields, the AESL crop quality team tested the soil in each 20- to 25-acre field in half-acre grids, collecting a soil sample from the center of each grid before the onions were planted to understand what the background growing environment is for each section of each field.

The team used a probe to measure the electrical conductivity signature of the soil in each field and worked with colleagues in UGA's Enterprise Information Technology Services and the Warnell School of Forestry and Natural Resources to survey the fields using drones with multiple types of sensors to create a visual, 2-D map of soil conditions in each field just before planting.

“We've got one field we're working on that has six different soil types within 25 acres, so it's really variable in terms of soil characteristics. We’re trying to figure how much that matters to onion pungency with the ultimate goal of developing management practices to suit all of those specific soil conditions,” Jackson said.

Once the onion plants were well-established and growing in each field, the EITS and Warnell team members used drones to measure the vegetative index to determine the health of plants in each field.

“Specific wavelengths of light can show you where the plants are healthy and where they are not as healthy within each field. This helps us to see if there’s an overlap between onion performance and soil characteristics based on those measurements,” Jackson said.

In a similar study in one producer’s field last year, researchers found that applying sulfur-containing fertilizers necessary for onion production produced hotter onions where the sandy layer of the soil was more shallow — 7 to 10 inches — before reaching the claypan. In areas where the claypan was deeper, approximately 30 inches below the sandy surface, the onions were much sweeter.

“So, if a field has a really shallow clay layer in one section compared to the rest of the field, we would make different recommendation to that grower for how to fertilize the onions so that you don't have that buildup of sulfur that then creates pungent onions,” Jackson explained.

This year, the team has expanded the study to three fields with significant variations in soil profile, plus a 25-acre control field with very uniform soil characteristics to compare how variability influences flavor or production of the onions. This work is supported by faculty from the Department of Crop and Soil Sciences, multiple AESL staff members, UGA Extension area onion agent Chris Tyson, and UGA Extension Agriculture and Natural Resources agents from Tattnall, Toombs, Evans, Emanuel, and Treutlen counties.

Past research has shown that many environmental conditions can affect the pungency of onions, including irrigation, soil temperature, and both nitrogen and sulfur application rate. However, sulfur application rate has been shown to be the leading factor.

“Sulfur is the No. 1 thing that influences onion flavor. Nitrogen and other environmental factors do play a role, but they don’t seem to be a consistent determining factor like sulfur,” Jackson said. “The crux of this is that we're trying to basically create a tool growers could use to characterize their soil if they want sweet onions. The whole point is to help them with a management program that will allow them to grow consistently sweet onions across the region regardless of their specific soil conditions.”

By studying soil types and the varying depths of soil types, the researchers can develop customized fertilizer programs for each producer and within each field to improve consistency of onion flavor within each crop.

“Ultimately I feel that we are going to end up with multiple classes of soil types and different fertilization procedures for each one of those soil types. Theoretically, this could lead to the application of precision agriculture technologies for providing variable-rate sulfur applications as a means of adjusting the amounts producers are applying based on the soil characteristics within each field,” Jackson said.

In May, the team went back to the test sites to collect onion bulbs from each grid in each field. They will test the nutrient profile of each sample to check levels of accumulated nutrients in the plant tissues and seed bolts, as well as measuring the lachrymatory-factor — the compound that causes your eyes to tear when you cut an onion — as well as the methyl thiosulfinates — the sulfur-containing compounds responsible for onion breath.

“When you eat an onion, those are the compounds that create the burning sensation in your mouth. They kind of have a metallic, grassy green flavor. All the kinds of things that stick with you,” Jackson said. “Those parameters give us a snapshot of how pungent or how intense the flavor is, with the ultimate idea of trying to find out if there's a correlation between soil conditions and onion flavor.”

Irrigation, soil temperature, and both nitrogen and sulfur application rate can affect the pungency of onions, but sulfur application is the leading taste factor.

Last year’s study showed a correlation between the quality, the yield and the production of these onions based on soil type. If this year’s tests show similar correlations, future research will be focused on how to manage production in each soil type.

“So, if you have shallow clay, how should we manage those areas versus the areas where you have deep clay. At that point we could potentially move into using precision agriculture techniques to control sulfur applications and get a more consistent crop,” Jackson said.

The research on Vidalia onions is supported by four south Georgia farms and funded through the Vidalia Onion Committee, a grower-founded organization dedicated to protecting and growing the Vidalia onion brand.

Currently, the use of precision agriculture is more prevalent in low-margin row crops — such as cotton or soybean — where the cost of inputs, such as fertilizer, to grow the crop with the best-possible yield is carefully balanced against the return on investment in the price producers can get for their crops at harvest. These producers are more likely to invest in precision agriculture technology, which can potentially save them money on inputs like fertilizer on very large farms.

“The reputation of Vidalia onions is built on the sweet flavor profile. Every time a customer buys a Vidalia onion, they expect a sweet onion," Lessl said. "If we can figure out ways to grow that onion with a consistent flavor profile, in the long run that strengthens the value of the brand.”

For more information on research at CAES, visit caes.uga.edu/research.

Maria M. Lameiras is a managing editor with the University of Georgia College of Agricultural and Environmental Sciences.

Source: UGA CAES Newswire